BGP Route Reflectors

Rules says that, routes learned via iBGP will never be advertised to another iBGP peer in normal conditions. One way to overcome this rule is to create a full mesh topology among iBGP peers. But in some conditions this is not feasible due to many reasons. So an alternative to BGP full mesh topology is Route Reflectors, which allows a router to advertise, or reflect, iBGP learned routes to other iBGP speakers. This route reflection reduces the number of iBGP peers within an AS.

In normal cases, maintain a full iBGP mesh among RTA, RTB and RTC within AS 100. If you utilize the RR concept, RTC can be elected as an RR. In this way, RTC has a partial iBGP peering with RTA and RTB. Peering between RTA and RTB is not necessary because RTC is an RR for the updates that come from RTA and RTB.
neighbor A.B.C.D route-reflector-client

The router with this command is the RR, and the neighbors at which the command points are the clients of that RR. In the example, the RTC configuration has the neighbor router-reflector-client command that points at the RTA and RTB IP address. The combination of the RR and the clients is a "cluster". In this example, RTA, RTB, and RTC form a cluster with a single RR within AS100.

Other iBGP peers of the RR that are not clients are "nonclients".


As AS can have more than one RR. In this situation, an RR treats other RRs just like any other iBGP speaker. Other RRs can belong to the same cluster (client group) or to other clusters. In a simple configuration, you can divide the AS into multiple clusters. You configure each RR with other RRs as nonclient peers in a fully meshed topology. Clients should not peer with iBGP speakers outside the client cluster.
Multiple Clusters

Consider the above diagram (Multiple Clusters). RTA, RTB and RTC form a single cluster where RTC is the RR. For RTC, RTA and RTB are clients and anything else is a nonclient. Remember that the neighbor route-reflector-client command points at clients of an RR. Similarly, RTD is the RR for clients RTE and RTF. RTG is an RR in the third cluster.

Note: RTD, RTC and RTG are fully meshed, but routers within a cluster are not. When an RR receives a route, the RR routes as list shows. However this activity depends on the peer type:
  1. Routes from a nonclient peer ▬ Reflects to all the clients within the cluster.
  2. Routes from a client peer ▬ Reflects to all the nonclient peers and also the client peers.
  3. Routes from an eBGP peer ▬ Sens the update all client and nonclient peers.
 Here is the relative BGP configuration of routers RTC, RTD, and RTB:
RTC#
router bgp 100
neighbor 2.2.2.2 remote-as 100
neighbor 2.2.2.2 route-reflector-client
neighbor 1.1.1.1 remote-as 100
neighbor 1.1.1.1 route-reflector-client
neighbor 7.7.7.7 remote-as 100
neighbor 4.4.4.4 remote-as 100
neighbor 8.8.8.8 remote-as 200
RTB#
router bgp 100
neighbor 3.3.3.3 remote-as 100
neighbor 12.12.12.12 remote-as 300
RTD#
router bgp 100
neighbor 6.6.6.6 remote-as 100
neighbor 6.6.6.6 route-reflector-client
neighbor 5.5.5.5 remote-as 100
neighbor 5.5.5.5 route-reflector-client
neighbor 7.7.7.7 remote-as 100
neighbor 3.3.3.3 remote-as 100

Because there is a reflection of the iBGP learned routes, there can be a routing information loop. the RR schemes has a few methods to avoid this loop:
  • Originator-ID: This is an optional, non-transitive BGP attribute that is 4 bytes long. An RR creates this attribute. The attribute carries the router ID (RID) of the originator of the route in the local AS. If, due to poor configuration, the routing information comes back to the originator, the information is ignored.
  • Cluster-list: A cluster list is a sequence of cluster IDs that the route has passed. When an RR reflects a route from the RR clients to nonclients outside of the cluster, the RR appends the local cluster ID to the cluster list. If this update has an empty cluster list, The RR creates one. With this attribute, an RR can identify if the routing information has looped back to the same cluster due to poor configuration. If the local cluster ID is found in the cluster list, the advertisement is ignored.


Redundancy with Multiple RRs within a Cluster

 Usually, a cluster of clients has a single RR. In this case the router ID of the RR identifies the cluster. In order to increase redundancy and avoid single point of failure, a cluster can have more than one RR. You need to configure all RRs in the same cluster with a 4-byte cluster ID so that an RR can recognize updates from RRs in the same cluster.
Multiple RRs within a cluster
In the above diagram (Multiple RRs within a cluster) in this section, RTD, RTE, RTF and RTH belong to one cluster. Both RTD and RTH are RRs for the same cluster.

Note: There is redundancy because RTH has fully meshed peering with all the RRs. If RTD goes down, RTH takes the place of RTD.

Here is  the configuration  of RTH, RTD, RTF, and RTC:
RTH#
router bgp 100
neighbor 4.4.4.4 remote-as 100
neighbor 5.5.5.5 remote-as 100
neighbor 5.5.5.5 route-reflector-client
neighbor 6.6.6.6 remote-as 100
neighbor 6.6.6.6 route-reflector-client
neighbor 7.7.7.7 remote-as 100
neighbor 3.3.3.3 remote-as 100
neighbor 9.9.9.9 remote-as 300
bgp cluster-id 10
 RTD#
router bgp 100
neighbor 10.10.10.10 remote-as 100
neighbor 5.5.5.5 remote-as 100
neighbor 5.5.5.5 route-reflector-client
neighbor 6.6.6.6 remote-as 100
neighbor 6.6.6.6 route-reflector-client
neighbor 7.7.7.7 remote-as 100
neighbor 3.3.3.3 remote-as 100
neighbor 11.11.11.11 remote-as 400
bgp cluster-id 10
RTF#
router bgp 100
neighbor 10.10.10.10 remote-as 100
neighbor 4.4.4.4 remote-as 100
neighbor 13.13.13.13 remote-as 500
RTC#
router bgp 100
neighbor 1.1.1.1 remote-as 100
neighbor 1.1.1.1 route-reflector-client
neighbor 2.2.2.2 remote-as 100
neighbor 2.2.2.2 route-reflector-client
neighbor 4.4.4.4 remote-as 100
neighbor 7.7.7.7 remote-as 100
neighbor 10.10.10.10 remote-as 100
neighbor 8.8.8.8 remote-as 200
 Note: You do not need the bgp cluster-id command for RTC because only one RR exists in that cluster.





Comments

Post a Comment

Popular posts from this blog

Specifying SSH port in Ansible Inventory

There may be some instances where you set a custom port for SSH on your network device. If ssh port for hosts is different than the default port 22, it can be specified in the inventory file with colon (:) after hostname. #vi inventory # Inventory file for Ansible   [XE] ios-xe-mgmt.cisco.com:8181 ios-xe-mgmt-latest.cisco.com:8181   [XR] sbx-iosxr-mgmt.cisco.com:8181
Read more

Ansible-Playbook to display output of multiple show commands (using stdout_lines with Loop)

 In this playbook, we we'll see how we can get display of multiple show commands in stdout_lines format. We can make use of loops (or with_items) for submitting multiple commands, but debug output with stdout_lines does not gives the formatted result as it would give for single command. So in case of multiple commands, we can debug the output of each command separately in stdout_lines format.
Read more

Filtering Routes in BGP using Route-maps and Prefix-list

Image
Order of preference of attributes in BGP The order of preference varies based on whether the attributes are applied for inbound updates or outbound updates. For inbound updates the order of preference is:     route-map     filter-list     prefix-list, distribute-list For outbound updates the order of preference is:     prefix-list, distribute-list     filter-list     route-map NOTE: The attributes prefix-list and distribute-list are mutually exclusive, and only one command (neighbor distribute-list or neighbor prefix-list) can be applied to each inbound or outbound direction for a particular neighbor. Scenario: We own the AS500 and advertising a network block of 192.0.2.0/24 and 180.179.179.0/16 to two different ISPs. Objectives: Configure router R1 to establish eBGP neighbor relationship with ISP1. Configure router R1 to establish eBGP neighbor relationship with ISP2. Advertise  192.0.2.0/24 ...
Read more

Ansible Playbook for Network OS Upgrade with pre and post checks

You have 100s of network switches or routers that you need to upgrade. How much time would it take for you to do the upgrades? There are a lot number of sub-tasks involved while upgrading IOS image of a Cisco router or a switch. This time of upgradation can be reduced through automation from various Enterprise Configuration Management tools that also have ability to upgrade network OS. Though these tools give an easy to use graphical interface, but this requires you to have appropriate license and also restricts you to customize your upgrade process.
Read more

VMware NSX Traffic Flow — East-West & North-South

Image
Understanding how traffic is flowing in NSX environment is an important aspect to successfully maintain and troubleshoot networks having NSX. In this post we'll understand hop-by-hop flow of traffic in East-West and North-South directions. East-West: VMs on Same Subnet, Same Host VM-1 has IP address 172.16.20.6 and VM-2 has IP address 172.16.20.7 VM-1 vNIC è Logical Switch (Segment ID 5002) è VM-2 vNIC
Read more

Bypassing Proxy Server in Google Chrome

Image
There may be times when you might want to use Google Chrome and do not wish to use the proxy settings for which you are bound to use in Internet Explorer because of your company policies. If you try to change proxy settings in Google Chrome, by default it will open the System Internet Properties dialogue box, and most of the times, change in proxy setting options is restricted by domain group policy. So here is an alternate way to remove or disable proxy settings in Google Chrome. 1. Right click the GoogleChrome.exe and create it's shortcut. 2. Open the properties of the shortcut of GoogleChrome.exe created in previous step. 3. Here you have to edit the Target of this shortcut. You have to add   --proxy-server=""  after the existing path. You are done. Now enjoy the web browsing without any restrictions.
Read more

Export or Backup Azure Network Security Groups into CSV using PowerShell

There could be many use cases where you may want to export Network Security Groups into CSV. You might have question, how to export or backup Azure Network Security Groups into CSV. Here is the PowerShell script that you can use to export Azure Network Security Groups into CSV using PowerShell script. This script will export Network Security Group along with rules of all Active subscriptions into a CSV.
Read more

Ansible-playbook for backing up running config of Cisco IOS

This ansible-playbook can be used to backup running configuration from Cisco IOS devices. You can refer to my earlier post Getting Started with your first ansible-playbook for Network Automation  to know about the parameters used in this playbook. Inventory file # Inventory file for Ansible [XE] ios-xe-mgmt.cisco.com:8181 ios-xe-mgmt-latest.cisco.com:8181 [XR] sbx-iosxr-mgmt.cisco.com:8181 [all:vars] ansible_network_os=ios Playbook --- - name: Define Parameters   hosts: XE   gather_facts: no   connection: network_cli   tasks:    - name: backup the config      ios_config:       backup: yes      register: backup_config    - name: Store the config to directory      copy:       src: "{{ backup_config.backup_path }}"       dest: "/tmp/backups/{{ inventory_hostname }}" Running the playbook [prashant@Prashant-VM01 ~]$ ansible-playbook play03.yml -i /home/prasha...
Read more