Ip routing Q&A

1.
Q: what are the administrative distances for routing protocols ?
A:
Directly connected route ------------ 0
Static route out an interface -------- 0
Static route to next-hop address ----- 1
EIGRP summary route ------------------ 5
External BGP ------------------------- 20
Internal EIGRP ----------------------- 90
IGRP --------------------------------- 100
OSPF --------------------------------- 110
IS-IS -------------------------------- 115
RIP ---------------------------------- 120
EGP ---------------------------------- 140
ODR ---------------------------------- 160
External EIGRP ----------------------- 170
Internal BGP ------------------------- 200
DHCP-learned ------------------------- 254
Unknown ------------------------------ 255

Notes:
An administrative distance of 255 will cause the router to disbelieve the route entirely and not use it.
Since IOS 12.2, the administrative distance of a static route with an exit interface is 1. Prior to the release of 12.2 it was in fact 0.

2.
Q: Can administrative distance be changed ?
A: You can modify the administrative distance of a protocol through the distance command. (CISCO)


3.
Q. What are private IP addresses?

A. The Internet Assigned Numbers Authority (IANA) has reserved the following three blocks of the IP address space for private internets (RFC1918)
10.0.0.0 - 10.255.255.255 (10/8 prefix)
172.16.0.0 - 172.31.255.255 (172.16/12 prefix)
192.168.0.0 - 192.168.255.255 (192.168/16 prefix)
4.
Q. well known reserved ipv4 multicast address ?

A. reserved for IP multicasting and registered with the Internet Assigned Numbers Authority (IANA)
224.0.0.0 Base address (reserved)
224.0.0.1 All systems on the same network segment
224.0.0.2 All routers on the same network segment
224.0.0.5 OSPF AllSPFRouters address. Used to send Hello packets to all OSPF routers on a network segment
224.0.0.6 The OSPF AllDRouters address. Used to send OSPF routing information to OSPF designated routers on a network segment
224.0.0.9 The RIP version 2 group address. Used to send routing information using the RIP protocol to all RIP v2-aware routers on a network segment
224.0.0.10 EIGRP group address. Used to send EIGRP routing information to all EIGRP routers on a network segment
224.0.0.13 PIM Version 2 (Protocol Independent Multicast)
224.0.0.18 VRRP
224.0.0.19 - 21 IS-IS over IP
224.0.0.22 IGMP Version 3 (Internet Group Management Protocol)
224.0.0.102 Hot Standby Router Protocol Version 2
224.0.0.251 Multicast DNS address
224.0.0.252 Link-local Multicast Name Resolution address
224.0.1.1 Network Time Protocol address
224.0.1.39 Cisco Auto-RP-Announce address
224.0.1.40 Cisco Auto-RP-Discovery address
224.0.1.41 H.323 Gatekeeper discovery address

5.
Q. what is unregistered multicast packet ?
A. RFC4541: An unregistered packet is defined as an IPv4 multicast packet with a destination address which does not match any of the groups announced in earlier IGMP Membership Reports.
If a switch receives an unregistered packet, it must forward that packet on all ports to which an IGMP router is attached. A switch may default to forwarding unregistered packets on all ports.
Switches that do not forward unregistered packets to all ports must include a configuration option to force the flooding of unregistered packets on specified ports.

6.
Q: ethernet header format ?


A.
Ethernet type II: Type at the type/Length
IEEE 802.3 Frame:
IEEE 802.3 with SNAP header
802.3 Raw: Length at type/length. Novell's non-standart Same as 802.3, without the IEEE 802.2 LLC header. Novell's IPX is the only protocol that uses the 802.3 raw frame type

7.
Q: how to identify Ethernet type ?
A:
1. if the Type/Length field has higher than (0x05Dc), then it is Ethernet II, the Type/Length is type and date is followed immeditately.
2.If it is length, if DSAP is 0xAA, then it has SANP header.
3. For 802.3 Raw: Novell decided to use the first two bytes in the data portion of the packet, the IPX checksum field, to identify an 802.3 raw frame using the IPX/SPX protocol. All LAN drivers would use the value 0xFFFF in these two bytes to designate the packet as 802.3 raw.
3.


8.
Q. well known ether type:
A: For detail, please check RFC5342
0x0000 - 0x05DC: IEEE 802.3 length
0x0800 : IP
0x0806: ARP
0x86DD: Ipv6

9.
Q. what is L2TP ?

A: The Layer 2 Tunnel Protocol (L2TP) is IETF standard that combines two existing tunneling protocols: Cisco's Layer 2 Forwarding (L2F) and Microsoft's Point-to-Point Tunneling Protocol (PPTP). L2TP is an extension to the Point-to-Point Protocol (PPP), which is an important component for VPNs.
Although L2TP acts like a Data Link Layer protocol in the OSI model, L2TP is in fact a Session Layer protocol,[2] and uses the registered UDP port 1701.


10.
Q. What is L2PT ?
A. Layer 2 protocol tunneling allows Layer 2 protocol data units (PDUs) (CDP, STP, and VTP) to be tunneled through a network.
Juniper EX switch support L2PT from 10.0.