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BGP RR query

CE1-PE1-RR-PE2-CE2

In an MPLS environment with above scenario I am learning 0.0.0.0/0 route from CE1 (As-path contains one AS) at PE1 and from CE2 (AS-path contains three AS) at PE2 . In this case route from PE1 will became best even at PE2.

PE2 has installed route 0.0.0.0/0 from PE1 as best one because it contains shortest AS-PATH

Does PE2 shows this 0.0.0.0/0 (Non-best route) in advertised route to RR. if not why?

Thanks in advance

1 ACCEPTED SOLUTION

Accepted Solutions
Hall of Fame Super Silver

Re: BGP RR query

Hello Mahesh,

>> Does PE2 shows this 0.0.0.0/0 (Non-best route) in advertised route to RR. if not why?

No, PE2 shouldn't advertise back 0.0.0.0/0 to RR because its best path is learned from RR itself.

See this as a split-horizon loop avoidance rule.

So a side effect of PE2 installing the route coming from PE1 is that PE2 stops to advertise the 0.0.0.0/0 received from CE2.

if you want PE2 to still advertise that prefix you need to use weight or local preference to make PE2 to prefer the eBGP path from CE2

Hope to help

Giuseppe

4 REPLIES

Re: BGP RR query

HI Mahes, [Pls RATE if HELPS]

If i clearly understand the Scenario;

Assuming Customer has got 2" CE Routers: CE1, CE2.

CE1 as PRY and CE2 as SEC on the same AS# system. Customer is doing the As-prepend for the Routes advertised from CE2 towards the Provider.

The Primary Function of the RR is to show the Best Path Route available in the Network. Since PE1 receives the Best route from CE1 the same is propagated to the other PE Routers via the RR.

In-case of CE1-PE1 connectivity fails; the secondary will be preferred as Best.

Hope I am Informative.

Pls RATE if HELPS

Best Regards,

Guru Prasad R

Community Member

Re: BGP RR query

Thanks guru,

But I want to know only one thing and it is does PE2 shows this 0.0.0.0/0 route in its advertised route of BGP running between PE2 and RR.

I know PE2 has already installed the best one from PE1.

Regards

Mahesh

Re: BGP RR query

HI Mahesh,

Assume PE1 receives the default 0.0.0.0/0 route from CE1 and the LP of 800 is set at PE1 for the received routes.

Also, PE2 receives the default 0.0.0.0/0 route from CE2 and the LP of 600 is set at PE2 for the received routes.

In the RR, the routes received from PE1 will be as Valid & Best nevertheless for the routes received from PE2 will be as Valid only.

Whereas in other PE Routers in the Service Provider Network; the Valid & Best route will be installed (in routing table) for usage.

As already informed, the PE1-CE1 connectivity fails; the Secondary default 0.0.0.0/0 will be preferred.

Hope I am Informative.

Pls RATE if HELPS.

Best Regards,

Guru Prasad R

Hall of Fame Super Silver

Re: BGP RR query

Hello Mahesh,

>> Does PE2 shows this 0.0.0.0/0 (Non-best route) in advertised route to RR. if not why?

No, PE2 shouldn't advertise back 0.0.0.0/0 to RR because its best path is learned from RR itself.

See this as a split-horizon loop avoidance rule.

So a side effect of PE2 installing the route coming from PE1 is that PE2 stops to advertise the 0.0.0.0/0 received from CE2.

if you want PE2 to still advertise that prefix you need to use weight or local preference to make PE2 to prefer the eBGP path from CE2

Hope to help

Giuseppe

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