Subject: Re: [bess] EVPN Multihoming and Symmetric IRB

From: <[email-masked]>
Date: Thu, 25 Apr 2019 08:58:39 +0800 (CST)

Hi Muthu and everyone else,

The IP Aliasing in Symmetric IRB is described in draft-sajassi-bess-evpn-ip-aliasing-00 .

It works well for each local host<IP,MAC> learned on the IRB interface.

I think when the IRB interface itself is configured with an anycast Gateway IP-address for its corresponding subnet,

the IP address of the IRB interface itself doesn't have an explicit ESI to be announced together with its own MAC/IP address in current documents,

Can we use an vESI for IRB interface itself or its corresponding MAC-VRF (which is similar ot the I-ESI concept)? 

This issue exists in both symmetric IRB and asymmetric IRB. 

Thanks

Bob

On Wed, 24 Apr 2019 09:57:51 +0530

Muthu Arul Mozhi Perumal <[email-masked]> wrote:

> Hi Everyone,

> draft-ietf-bess-evpn-inter-subnet-forwarding does not explicitly describe

> the implications of EVPN multihoming on IRB. It seems to assume that the

> IRB procedures can be easily extrapolated to multihoming following RFC 7432

> and it says so at least for the mobility procedures described in section 4.

> However, I think there are key implications of multihoming for symmetric

> IRB.

> Fast Convergence:

> Section 8.2 of RFC 7432 says the following:

> <snip>

>    Upon a failure in connectivity to the attached segment, the PE

>    withdraws the corresponding set of Ethernet A-D per ES routes.  This

>    triggers all PEs that receive the withdrawal to update their next-hop

>    adjacencies for all *MAC addresses* associated with the Ethernet

>    segment in question.  If no other PE had advertised an Ethernet A-D

>    route for the same segment, then the PE that received the withdrawal

>    simply invalidates the *MAC entries *for that segment.  Otherwise, the

>    PE updates its next-hop adjacencies accordingly.

> </snip>

> Clearly, it describes fast convergence only for the MAC addresses of TSs

> (and not for their IP addresses). In symmetric IRB, the ingress PE performs

> a route lookup for the destination TS prefix in IP-VRF and forwards the

> packet to the egress PE. Hence, the above fast convergence is not

> applicable. It however is applicable for asymmetric IRB where the

> destination subnet is configured in the ingress PE and it performs a lookup

> in the BT corresponding to the destination subnet and forwards the frame.

> Aliasing and Backup Path:

> With symmetric IRB, the ingress PE cannot use alias label (i.e. label

> advertised in AD per EVI route) to load balance traffic sent to egress PEs

> multihomed to the same CE, since the egress PE needs to first perform a

> route lookup for the destination prefix in the IP-VRF to forward the

> packet. The ingress PE instead needs to rely on multipath techniques

> applicable to L3VPN (such as BGP multipath).

> Now, coming to the backup path, section 8.4 of RFC 7432 says the following:

> <snip>

>    The backup path is a closely related function, but it is used in

>    Single-Active redundancy mode.  In this case, a PE also advertises

>    that it has reachability to a given EVI/ES using the same combination

>    of Ethernet A-D per EVI route and Ethernet A-D per ES route as

>    discussed above, but with the "Single-Active" bit in the flags of the

>    ESI Label extended community set to 1.  A remote PE that receives a

>    MAC/IP Advertisement route with a non-reserved ESI SHOULD consider

>    the *advertised MAC address* to be reachable via any PE that has

>    advertised this combination of Ethernet A-D routes, and it SHOULD

>    install a backup path for that MAC address.

> </snip>

> Clearly, it describes the backup path only for the MAC address of the TS

> (and not for their IP address). Hence, it is not applicable to symmetric

> IRB. It however is applicable to asymmetric IRB.

> Is my understanding correct? Shouldn't the implications of multihoming on

> symmetric IRB be explicitly captured

> in draft-ietf-bess-evpn-inter-subnet-forwarding?

> Regards,

> Muthu

---

From: Muthu Arul Mozhi Perumal <[email-masked]>
Date: Thu, 25 Apr 2019 11:38:07 +0530

Hi Bob,

Thanks for your response. Please see inline..

> It works well for each local host<IP,MAC> learned on the IRB interface.
Glad to know that my understand that fast convergence, aliasing and backup
path as described in RFC 7432 is applicable only for host MAC addresses and
not for their IP addresses is correct :) I think it is important to capture
this implication for symmetric IRB
in draft-ietf-bess-evpn-inter-subnet-forwarding and probably add a
reference to draft-sajassi-bess-evpn-ip-aliasing to indicate how those
functionalities can be realized in the symmetric IRB case.

On a different note, when IP aliasing is used, I think it slightly modifies
the definition of symmetric IRB because the egress PE is no longer doing a
lookup in the IP-VRF, rather forwarding the packet on the ES after doing a
LFIB lookup. This should perhaps be captured in
draft-sajassi-bess-evpn-ip-aliasing.

>
> I think when the IRB interface itself is configured with an anycast
> Gateway IP-address for its corresponding subnet,
>
> the IP address of the IRB interface itself doesn't have an explicit ESI to
> be announced together with its own MAC/IP address in current documents,
>
Why do we need a non-zero ES for the anycast gateway IP address? My
understanding is that you include the gateway IP address in the MAC/IP
advertisement route for MAC address aliasing so that the receiver can check
if the received IP address matches with the locally configured anycast
gateway IP address. Does it serve any other purpose?

Regards,
Muthu