ASR Router with BGP Neighbours and Failover

[mahesh18]

We have ASR Routers with 3 BGP neighbour's which are 3 different ISP's.

We are learning routes from all of them

From ISP 1 we are learning full Internet Routing table.

IF we lose physical link to ISP 1 will our Router learn all the routes from ISP 2 and 3?

We do not have any link monitoring configured on physical links connecting to the 3 ISP's

[mahesh18]

Just saw your note now  yes we are receiving full routing table from the ISP.

[Dennis Mink]

on your router you will need to tweak the metrics from all ISPs, so say you receive 3 default routes from 3 different provider you will need to set up a route map for each ISP to tweak the local preference metric.

[mahesh18]

any config or command i can use to check if metric is tweaked or not?

[Giuseppe Larosa · Accepted solution]

Hello Mahesh,

just look at the output of show run | begin router bgp

if you see commands that sets local preference per neighbor  or any route-map applied in direction in per neighbor some form of manipulation is configured otherwise simply BGP uses the default order of criteria to choice the best path.

Hope to help

Giuseppe

[mahesh18]

I run that command do not see any local preference or route map with Neighbour command.

Only thing i see with one Neighbour which is ISP 2  is ebgp-multihop 2

I only see route map config

route-map set-as-prepend permit 10
set as-path prepend 16569 16569 16569
!
route-map set-local-pre permit 10
set local-preference 150
!
route-map AllowList permit 10
match ip address 80

This is not applied to any nei command.

when i check show route-map all
STATIC routemaps
route-map set-as-prepend, permit, sequence 10
Match clauses:
Set clauses:
as-path prepend 16569 16569 16569
Policy routing matches: 0 packets, 0 bytes
route-map set-local-pre, permit, sequence 10
Match clauses:
Set clauses:
local-preference 150
Policy routing matches: 0 packets, 0 bytes
route-map AllowList, permit, sequence 10
Match clauses:
ip address (access-lists): 80
Set clauses:
Policy routing matches: 0 packets, 0 bytes
DYNAMIC routemaps
Current active dynamic routemaps = 0

this confirms that prefix preference is configured right?

[Giuseppe Larosa · Accepted solution]

Hello Mahesh,

the route-maps are not applied to any neighbor so no type of route manipulation is occurring in this moment in your router.

You have a route-map that could be used to increase the local-preference to 150 for all received routes if you add a command like

neighbor <IP-address-ebGP-neighbor> route-map set-local-pre in

under router bgp or router bgp address-family ipv4-unicast

The use of a command like the one proposed above should be considered only for the eBGP neighbors that are sending you partial BGP routes like ISP2 and ISP3.

As I have written in another thread I would give local preference 200 to ISP3 that gives you a small number of routes, then local preference 150 to ISP2 that sends you more routes. You can leave ISP1 and the iBGP neighbor with default settings of local-preference 100.

However, you need to consider the current link usage of links towards ISP3, ISP2 and ISP1 and their effective speed that can be different. Likely ISP1 has a link with higher bandwidth.

An higher local-preference can make a prefix with a longer AS path preferred because local preference is considered before AS path length in BGP best path selection.

At the moment as noted by Francesco no form of route manipulation is occurring in your router.

Hope to help

Giuseppe

[Francesco Molino · Accepted solution]

Based on your previous output, there is no local preference or whatsoever. The route chosen is based on as-path.