Juniper-关于双层dhcp-relay结构问题分析.docx
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1、Juniper-关于双层dhcp-relay结构问题分析Juniper双层dhcp-relay结构问题分析一、拓扑图说明:如上图,PC-1和PC-2都配置自动获取IP,但PC-1获取IP是在MX-1上做的DHCP中继,而PC-2是在R5上做的DHCP中继。二、配置文件 1、R1配置interface FastEthernet0/0 ip address 12.1.1.2 255.255.255.0service dhcpip dhcp pool vlan-2 network 172.17.17.0 255.255.255.0 default-router 172.17.17.1ip dhcp
2、pool vlan-121 network 172.20.3.0 255.255.255.0 default-router 172.20.3.1ip route 0.0.0.0 0.0.0.0 12.1.1.1 2、R5配置interface FastEthernet0/0 ip address 172.16.16.2 255.255.255.0interface FastEthernet1/0 switchport access vlan 2interface Vlan2 ip address 172.17.17.1 255.255.255.0 ip helper-address 12.1.
3、1.2ip route 0.0.0.0 0.0.0.0 172.16.16.13、R2配置interface FastEthernet0/0 ip address dhcp4、R6配置interface FastEthernet0/0 ip address dhcp 5、MX1配置 interfaces ge-0/0/1 unit 0 family inet address 12.1.1.1/24; ge-0/0/4 unit 0 family inet address 172.16.16.1/24; ge-0/0/5 unit 0 family bridge interface-mode a
4、ccess; vlan-id 121;forwarding-options server-group dhcp-1 12.1.1.2; active-server-group dhcp-1; group group-1 interface irb.121; routing-options static route 172.17.17.0/24 next-hop 172.16.16.2; bridge-domains vlan-121 vlan-id 121; routing-interface irb.121;三、问题描述通过上述配置后,则R2能正常获取IP,但PC-2则无法获取。1、在PC-
5、2 debug信息2、MX-1查看dhcp relay包统计四、故障处理 通过上述抓包和MX-1上dhcp relay包统计可以看出,在MX-1上对dhcp relay过来的单播包进行了丢弃处理,这个是juniper设备独有的特性,其他厂商不需要额外处理,但 juniper自身作为dhcp relay后,则默认对dhcp relay过来的单播报文作丢弃处理。若需要接受这种报文,则需要开启。forwarding-options dhcp-relay forward-snooped-clients all-interfaces;通过如上命令后,R6才可以正常获取到IP地址。 英文解释(http:/
6、blog.ip.fi/)There are two different ways to configure DHCP in Junos, bootp helper and dhcp relay. These work in very different manner, bootp helper is being phased out and is not supported for example in QFX10k. Behaviour of bootp helper is obvious, it works like it works in every other sensible pla
7、tform. Behaviour of dhcp-relay is very confusing and its not documented at all anywhere. If its possible in your platform to configure bootp helper, do it. If not, complain to Junos about dhcp-relay implementation and ask them to fix it. The main problem with dhcp-relay implementation is that once y
8、ouve configured it, youre punting all dhcp traffic in all interfaces. Normal transit traffic crossing your router is subject to this punt, so transit customers will experience larger jitter and delay of packets being punted and almost certainly reordering, because the non-dhcp packet that came after
9、 but was not subject to punt will be forwarded first. Technically reordering does not matter, as long as it does not happen inside a flow, but its not desirable. How the sequence of operation works in Junos for dhcp-relay: 1. Transit packet touches ingress NPU 2. After L2 lookup, before L3 lookup in
10、gress NPU punts the transit packet to PFE CPU 3. PFE CPU, for reasons obvious to people on drugs encapsulates the transit DHCP packet with new IP, UDP and 28magic bytes header. ip_new, udp_new, 28_bytes, ip_old, udp_old, payload 4. PFE CPU sends the encapsulated DHCP packet to RE, so that JDHCPD can
11、 inspect it 5. JDHCPD determines if its relevant to it or not, in our case its not, in normal configuration it proceeds to drop the transit packet as it was not interesting to us! I dont disagree that on some instances it may be desirable to snoop transit DHCP messages, to see what server unicasts t
12、o our client, but that is small percentage of the traffic. We know the DHCP servers we have, why cant we decide what is punted and what is not? If operator is doing her due diligence and has exact lo0 filter which only allows things to enter control-plane which are actually needed there, all of this
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- Juniper 关于 双层 dhcp relay 结构 问题 分析
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